Many devices use touch screens as a convenient and intuitive way for users to both view and enter information. Common applications include mobile phones, PDAs, ATMs, GPS navigation systems, electronic games, and computer interfaces, to name just a few examples. Touch screens allow a user to interact with a device by using a finger or stylus to touch objects displayed on a screen, such as icons, text, buttons, etc. In some applications, a user may also “write” directly on a touch screen, such as in a PDA or other device that implements character recognition.
There are numerous technologies used to implement touch screens, including technologies that use the electrical property of capacitance to detect user inputs. A capacitive touch screen sensor is one type of sensor that generally operates by capacitive coupling of current through a transparent dielectric layer to a user's finger (or stylus). This type of sensor typically includes a capacitive sensing circuit with multiple transparent electrodes, each producing an electric field across a touch sensitive area of the sensor. The capacitive sensing circuit may be adjacent to a transparent sensor substrate (e.g., glass). A touch near one or more electrodes of the sensing circuit may affect the electric field and create a signal that can be detected. A set of electrical connections may be made between the sensing circuit and detection electronics (e.g., a controller) that resolves the signals to determine the location of the touch on the sensor. The coordinates of the location may then be communicated to another processor such as a host computer for further processing.
In a typical capacitive sensor, a stack that comprises a plurality of transparent layers is utilized, including substrate layers (e.g., glass), transparent conductive layers (e.g., indium tin oxide (ITO)) adjacent to the substrates, and possibly a layer acting as a shield on the bottom of the stack. Metal traces and flexible printed circuit (FPC) connectors may be used to couple the conductive layers to the detection electronics (e.g., a controller).
FIGS. 1A-1B illustrate one type of prior art capacitive touch panel 10 that includes top and bottom glass substrates 12, 14 that are each coated with ITO layers 16, 18, respectively. The ITO layers 16, 18 each include a pattern of electrodes positioned across a surface of the top and bottom glass substrates 12, 14. As shown, the top and bottom glass substrates 12, 14 are arranged in a face-to-face ITO pattern structure, such that the surfaces of the top and bottom glass substrates 12, 14 coated with the ITO layers 16, 18 are facing each other. The top and bottom glass substrates 12, 14 may be bonded together by an optically clear adhesive (OCA) 20, such as an OCA sold by 3M Electronics.
The pattern of electrodes in both the top and bottom ITO layers 16, 18 may be coupled to metal traces 22, 24 to connect the ITO electrodes to touch detection electronics such as a controller. For instance, the metal traces 22, 24 may be positioned at opposite edges of the top and bottom glass substrates 12, 14 with portions thereof in contact with the ITO layers 16, 18. Two flexible printed circuit (FPC) connectors 26, 28 may then be connected to the metal traces 22, 24 by a bonding process for instance. The FPC connectors 26, 28 are discussed in further detail below in reference to FIG. 4.
FIG. 1B is similar to FIG. 1A but additionally illustrates a hot bar 30 that may be used in a process for bonding the FPC connector 28 to the metal traces 24 associated with the bottom substrate 14. As shown, the hot bar 30 may be pressed onto the surface of the FPC connector 28 to urge the FPC connector 28 against the metal traces 24 for a predetermined period of time (e.g., several seconds). To perform this bonding process, there must be sufficient clearance above the metal traces 24 to place the hot bar 30, as well as a pressing mechanism (not shown). As can be appreciated, the metal traces 22 positioned on the lower surface of the top substrate 12 may be on a side of the panel 10 different from the metal traces 24, so that there will be sufficient clearance for the hot bar 30 and pressing mechanism to secure the FPC connector 26 to the top ITO layer 16 via the top metal traces 22. However, the use of two FPC connectors 26, 28 increases cost and the complexity of the manufacturing design and process.
To eliminate the need for two separate FPC connectors, the prior art touch panel 40 illustrated in FIG. 2 may be used. Similar to the panel 10 shown in FIG. 1, the panel 40 includes top and bottom glass substrates 42, 44 coated with respective top and bottom ITO layers 46, 48 and adhered to each other by an OCA 50. In this touch panel however, the bottom ITO layer 48 is coated onto the lower surface of the bottom substrate 44, rather than the upper surface as shown in FIG. 1, such that the ITO pattern structure is in a “face-to-back” configuration. This design has the advantage of permitting a single bifurcated FPC connector 56 (a top view of FPC connector 56 is shown in FIG. 3) to be used, instead of two FPC connectors positioned on different sides of the touch panel, as in the panel 10 shown in FIG. 1. While the single FPC connector 56 may be used because there is sufficient clearance above the metal traces 52, 54 for the bonding process, this design also possesses significant disadvantages. For example, the design for the FPC connector 56 is complex because it must be operable to connect to metal traces 52, 54 in two different planes. Further, the FPC connector bonding process for the panel 40 may be complicated because two separate hot bar assemblies (e.g., each being similar to the hot bar 30 shown in FIG. 1B) are required to bond the bifurcated FPC connector to the metal traces 52, 54. Further, due to the face-to-back configuration, the bottom ITO layer 48 is significantly further away from a user's touch (e.g., using a finger or stylus) than the top ITO layer 46. This difference results in reduced signal strength from the electrodes on the bottom ITO layer 48, which in turn reduces the performance of the touch panel. Stated otherwise, increasing the distance between the top and bottom ITO layers 46, 48 reduces the capacitance of the touch panel. To compensate for the lower signal strength, an ITO layer that has a relatively low resistance may be used, but this has the negative effect of increasing the visibility of the ITO layer, thereby reducing the contrast of the display device.
FIG. 3 illustrates a top view of the bifurcated FPC connector 56 also shown in FIG. 2. When positioned in a touch panel, a first set of conductive pads 58 may be coupled to the metal traces 52 associated with the top ITO pattern 46, and another set of pads 60 may be connected to the metal traces 54 associated with the bottom ITO pattern 48. As can be appreciated, the other end of the connector 56 may include a plurality of pads 62 that may be coupled to touch detection electronics such as a controller.
FIG. 4 illustrates a top view of the normal (e.g., not bifurcated) FPC connector 26 that is also shown in FIGS. 1A-1B. As shown, the construction is relatively simple and includes conductive pads 27 that may be coupled to the metal traces 22 of the top ITO pattern 16 and a pad of connectors 29 that may be coupled touch detection electronics. Although the design and construction of the FPC connector 26 is relatively simple, the need for two FPC connectors on different sides of the touch panel 10 is costly, increases complexity of manufacturing, and increases the surface area required for the touch panel.